SE448864B - SET TO PREPARE A WATER SOLUTION SUITABLE FOR PREPARATION OF CHLORIDE Dioxide AND SIMILAR REDUCTION OF CHLORINE CONTENT IN A GAS SHELLED CHLORO Dioxide stream - Google Patents

Description

448 864 Commercially these reactions take place by continuously feeding the reactants into a reaction vessel and continuously removing the chlorine and chlorine dioxide produced therein from the reaction vessel.

A single reactor process for the production of chlorine dioxide is described in the US. PS 3 563 702, the teachings of which are to be incorporated in the present description, wherein alkali metal chlorate, an alkali metal chloride and a mineral acid solution are continuously fed to a single reactor, which constitutes both generator, evaporator and crystallizer, in amounts sufficient to form chlorine dioxide and chlorine at a temperature of about 50 - 1000 C and an acidity of about 2 - 5 - N or higher in the presence of a catalyst, or about 4 - 12 - N without catalyst, removal of water by vacuum-induced evaporation at about 100 - l40 mm Hg absolute, with concomitant removal of chlorine dioxide and chlorine, crystallization of the mineral acid salt in the generator and removal of the crystals from the reactor. Such a system is commercially available under the name SVP process, from Hooker Chemicals & Plastics Corp.

When the reaction takes place in the generator and in reactions where sulfuric acid is used as mineral acid reactant, crystals of sodium sulphate and sodium hydrogen sulphate precipitate in amounts which depend on the acid concentration and deposit on the bottom of the generator, from which they are withdrawn in the form of a slurry.

In addition to the use of sulfuric acid, hydrochloric acid can also be used as a mineral acid reactant, in which case the crystals removed from the generator are alkali metal chloride crystals, which product is often less desirable than alkali metal sulfate. Sodium sulphate is a valuable by-product, which can be used in the production of kraft pulp, which also applies to chlorine dioxide. Therefore, systems that produce chlorine dioxide and sodium sulfate are particularly useful, in that on-site coordination can be achieved with pulp treatment operations, using both the primary chlorine dioxide product and the recovered sodium sulfate in the pulp process, and especially in kraft pulp processes. lr 448 864 3 In some cases, however, the requirement for sodium sulphate is greatly reduced or eliminated. In certain types of pulping processes, no sodium sulphate is needed. In some kraft pulp processes, the need for sodium sulphate can be reduced or varied and the deposition of excess salt creates problems depending on the environmental protection regulations that currently apply. While the requirement for reduced quantities of sodium sulfate may vary, the need for chlorine dioxide remains.

In cases where reduced quantities of or no sodium sulphate are needed, the single reactor process can be converted for the use of hydrochloric acid as mineral acid reactant, in which case the by-product becomes sodium chloride. However, such systems are not as efficient as the systems that use sulfuric acid. Furthermore, only sodium chloride is produced and in cases where varying quantities of sodium sulphate are required, a generation of the desired amount of sodium sulphate would necessitate reversals back and forth from a catalyzed sulfuric acid system to a catalyzed hydrochloric acid system, with all the problems that arise. .

The present invention can be utilized in any conventional chlorine dioxide generating process using a chloride reducing agent, wherein chlorine is also produced. Examples of systems are the SVP -II process as well as the R-2 process and the Kesting process. These commercial processes conventionally produce a mixture of chlorine dioxide and chlorine. The advantage of using C102 instead of C12 in e.g. mass bleaching is that this gives high brightness with small losses in fiber strength. At present, all existing commercial processes produce Cl2 in varying amounts together with C102. Some separation occurs in the C102 washing towers used in such devices, since C102 is significantly more soluble in water than C12. The resulting effluent from the washing tower or scrubber contains approximately 8 g per liter of C102 and 1.5 - 2 g per liter of C12. Excess C12 is recovered from the top of the washing tower and fed to a caustic scrubber. However, if too much C12 is present in the C102-H20 solution, the pulp fiber strength is very severely affected. De l - 2 g. per liter of chlorine currently obtained is at the limit in this respect, and a more efficient separation of C102 from C12 would be desirable if possible.

Various methods for the selective removal of Cl2 from C102 and vice versa have been developed over the years. 1936 was taught according to the US. PS. 2,036,311 that oxides, hydroxides and carbonates of alkali metals or alkaline earth metals in the presence of water selectively absorbed C12.

US. PS 2,078,045 taught that continued chlorination of calcium oxide resulted in the formation of calcium chlorate, which when treated with HCl formed C102.

US. PS. 2,108,976 taught that when a C102-C12 gas mixture was bubbled through aqueous H2SO4, Cl2 was selectively absorbed, which can later be recovered by air evaporation.

Similarly, when C102-C12 is bubbled through dilute aqueous HCl, C102 is selectively absorbed.

US. PS 3,063,218 teaches that C102 is selectively absorbed from C12 when contacted with silica gel at temperatures above 30 ° C to form a stable mixture of chlorine dioxide and silica gel. The chlorine dioxide can be desorbed by increasing the temperature and evaporation with air. By US. PS. 2,481,241 teaches that C102 can be purified by adding a sufficient amount of SO2 for reaction with C12. According to the US. PS 2,871,097 also describes the countercurrent reaction of ClO2-Cl2 gas mixtures with an equimicurial solution of NaClO3 and NaCl2. _ All of these processes would yield purified C102. However, no one seems to be practically feasible from an operational point of view or financially. At present, there is no need to obtain pure C102, nor is there any need to eliminate C12 production completely. It is only necessary to reduce the current 1-2 grams per liter of C12 in the C102 solution currently obtained. Furthermore, since in mass production C12 is usually used in the bleaching processes, it is unnecessary to go to such an extreme to completely eliminate C12 production.

It has now been found in accordance with the present invention that an effective removal of chlorine from a gas mixture of chlorine and chlorine dioxide can be achieved by passing the gas mixture countercurrently through a washing or scrubbing solution containing sodium chlorate. sodium chloride and sodium hydroxide. The most effective use of this technique arises when the ratio of chlorate, chloride, and hydroxide is such that the yield after reaction; with chlorine in the gas mixture a product stream approaches the composition of the R-2 feed to the chlorine dioxide generator. Ideal operating conditions also imply a near neutral scrubber solution, i.e. pH 6 - 8, and a temperature in the order of about 60-800 ° C, although temperatures between about 50 and about 1000 ° C and a pH from about 4 to about 9 may be useful. The present invention provides a continuous in contrast batch process for the reduction of chlorine in a chlorine dioxide stream, which process consumes all the alkali present in the scrubber solution.

The accompanying drawing shows a schematic flow chart illustrating various aspects of the present invention.

In the drawing, 10 denotes the chlorine dioxide reactor in general, which may be any reactor suitable for the production of chlorine dioxide. The chlorine dioxide reactor shown functions as a generator, evaporator and crystallizer according to the single reactor process. Connected to the reaction vessel 10 is a circulation line 42 for the reaction solution, which is equipped with a heat exchanger 44, oxygen inlet means 46, chlorate reactant inlet 48, which can also function as a feed point for a reducing agent such as an alkali metal chloride, outlet 50 for removal alkali metal salt crystals and pump 52.

The reaction solution is continuously pumped through the loop 42, in which the reactant concentration and the temperature of the reaction solution are controlled and the solid product is removed.

The slurry of solid crystalline alkali metal sulfate, which is removed at the outlet 50, is passed via line 54 to the separator 56 for separating solid and liquid material, which may be of any device known per se, but 448 864 6 which here is illustrated in the form of a rotating filter. A cyclone separator, not shown, can be advantageously inserted into the conduit 54 between the filter 56 and the outlet 50 for classifying and thickening the slurry of solid alkali metal sulfate before it is fed into the filter. Thus, a certain amount of the fine fraction can be separated before the filtration and in combination with the filtrate and the washing water from the filter 56 returned via the line 58 to the inlet 48 on the loop 42. In any case the washing water and the filtrate from the filter 56 are returned via the line 58 and loop 42 to the bulk of the reaction solution in reactor 10. The product, ie neutral alkali metal sulfate, is removed from the filter 56.

The product vapor, containing up to about 10% chlorine dioxide, chlorine and water vapor, leaves the reactor 10 via line 60 to the cooler 62, in which water vapor condenses and the chlorine dioxide / chlorine mixture is rapidly cooled to a temperature below its decomposition temperature. The condensate is removed from the cooler 62 via line 64 and circulated to line 80 for supply to the absorber 82. In this way, only gases are fed to the chlorine dioxide scrubber for treatment. Gaseous chlorine dioxide and chlorine leave the cooler 62 via line 70 and are fed to the lower part of the chlorine dioxide scrubber 72. The chlorine in the gaseous mixture is reacted with the liquid countercurrent sodium chloride / sodium chlorate / sodium hydroxide solution introduced via line 66 according to reaction formula 6: 3Cl2 + NaOH - α> NaClO 3 + 5 NaCl + 3H 2 O The salt mixture fed through line 66 is proportioned to contain an approximately stoichiometric amount of sodium hydroxide for reaction preferably with chlorine to give chlorine dioxide with minimal chlorine content. The ratio of sodium chlorate to sodium chloride in the mixture is selected so that a product stream composition is obtained which approximately corresponds to the composition of the chlorate reactant feed to the reaction loop 42 through line 48. In addition, it is desirable to maintain the sodium chloride concentration close to the saturation concentration. above about 75% and preferably above about 90% of the saturation concentration, in order to minimize the solubility of the chlorine dioxide in the scrubber solution, while at the same time minimizing the chlorate concentration to the extent necessary to avoid crystallization in the scrubber. The desired reaction is achieved by operating the scrubber system at a temperature in the range 60 DEG-800 DEG C. and with the pH of the scrubber solution close to neutral, i.e. between pH 6 and pH 8, under which conditions the formation of chlorate from HOCl and NaOCl is maximized. A holding tank 76 may be necessary to achieve complete conversion of OCl_ to ClO3_. The reaction product from the scrubber reaction leaves the scrubber 72 via line 74 to a possible holding tank 76 and is then led via line 78 back to the circulation line 42 for the reaction solution. For convenience, line 78 is shown as if it were connected to line 48 together with the chlorate reactant inlet, although of course other arrangements are conceivable.

The chlorine dioxide, now substantially completely free of gaseous chlorine, leaves the scrubber 72 via line 80 to the ClO2 absorber 82, where it is combined with the water condensate from line 64. Cooled water is fed to the upper part of the absorber through line 81 and removed from the bottom of the absorber as a chlorine dioxide solution via line 84 for use.

Care must be taken when choosing the proportions of the brine to avoid crystallization problems with sodium chlorate in the scrubber reactor. However, as previously mentioned, it is desirable to select the proportions of chlorate, chloride and hydroxide in the scrubber solution so that a product solution having approximately the same composition as the original reactant is obtained, i.e. the input to the chlorine dioxide generator. Ideally, the reactant feed has approximately the same composition as an R-2 solution, with a chloride-to-chlorate ratio of about 0.7: 1-5: 1. Depending on the activity of the reaction solution in the combined generator-evaporator crystallizer used in the preferred embodiment and the temperature of the reaction solution, the use of feed solutions in which the molar ratio of chloride to chlorate is close to 0.7: 1 or 5: 1 lead to precipitation of sodium chlorate and sodium chloride, respectively, which is recovered together with the alkali metal salt of the acid used. Furthermore, feeding a solution to the reactor with a chloride-to-chlorate ratio of less than 1: 1 may require the addition of chloride to achieve efficient operation. This can be avoided by changing the composition of the input solution. For the production of chlorine dioxide, the use of feed solutions with a molar ratio in the order of 1: 0: 1 - 1, 3: 1 is preferred.

At this ratio, the acid normality of the feed reactant system is preferably maintained at about 3-4, although most preferably a normality of about 3.4-3.8 should be maintained. Furthermore, it is preferred to maintain the chloride ion molar at about 0.5 - 2 and the chlorate ion molar at about 1 - 2.

To illustrate the invention, the following examples are presented, which, however, are not to be construed as constituting any limitation of the invention.

Examples 1-4 A NaClO2 type laboratory scale C102 generator was used to give a mixture of C102, Cl2 and air. This mixture was further diluted with approximately 2100 m1 of additional air per minute. The gas mixture was led into the bottom of a 27: l27 mm high x 22 mm diameter glass tower, packed with 3.2 mm glass spirals. At the same time, a hot solution of sodium chloride / sodium chlorate / sodium hydroxide was passed downstream through the glass tower to wash the gas mixture. The composition of the gas feed was analyzed for both chlorine and chlorine dioxide both before and after scrubbing. In addition, the composition of the spent scrubber solution was determined with respect to both NaClO and NaClO 3. In addition, the temperature of the scrubber solution and the pH of the spent solution were measured. The results of these examples are illustrated in the table, where the improvements in the chlorine dioxide content that remain in the scrubbed gas stream are clear. .ml- 448 864 uem fl mmn wuu fl az N fl u pm fifi mamm fiflfi u> ß show H www n fl ëx fi a QUHN mo .uws fl wma wøwmwu: «« nwnsoaox H mš fl uwm flfl dwßmwuk Unwkfwmšw ww wm m ww mm m momz Homz mo fi umz Hwm., ¶ ,. w. @ | @ wmm om mm «m owøcß mm.ø ~ ø. ° m« .ø ø ~ .u mm u mw fl nmwmm fl_ ~ H | om az mN øm wm øwaoß mm_o ~ U ~ om «_ø @ ~ .ø > _møm.H u Nw fi æmwmm ~ _ß | mw mmßw ~ .m ßw mv owxøß wmm_o wmH ~ c mmm.ø mmm.ø> .H m Hw fi ßwwmm N.NH az mm wwfow> m »~« oß H «.o mc ~ c øm.ø wN.ø ß.mww + m 4 mm fi umwmm _ .¶. N. N _ N N ,. _ M o fi u Hu o fi u Hu mm o fi omz wmmo fi umz msHø »» m> ».mawv .n fifi mnqc .un.xm H \ m .nppmmcmeamw uwpww wnmu | w» w fi u @ ... ps ¶ fifl | \ = m @ H: maa .nmmåuvmkwu umxdu fl nmm NOAU w mm: Noummm. AW * ÉQEMHWV .Hwmdw 1 _ d: tvumbu .HQMMÉB 448 864 lo Example 5 A laboratory-type chlorine dioxide generator was used to produce chlorine dioxide and chlorine as illustrated in Examples 1 - 4. After equilibrium was reached, the following product flows and approximate material balances resulted.

A gas stream containing 0.20 g per minute of C12 and 0.45 g per minute of C102 was contacted in the C102 scrubber with 3.5 ml per minute of washing solution containing 283 g per liter of NaClO3 (2.1, 8%), 71 g per liter NaCl (5.6%), 78 g per liter of NaOH (6.0%) and 868 g per liter of H 2 O (66.6%). The wash solution thus provided 0.27 g per minute NaOH, 0.25 g per minute NaCl, 0.98 g per minute NaClO 3 and 3.00 g per minute H 2 O. When operating at a pH of 7 and at a temperature of 70 - 800 C, the composition of the effluent gas was 0.33 g per minute C102 and 0.02 g per minute C12, which corresponds to a 90% removal of C12 with a 73% recovery of C102.

The composition of the wash solution at the outlet was 0.52 g per minute NaCl (10.8%), 1.23 g per minute NaClO 3 (25.6%) and 3.06 g per minute H 2 O (63.6%). The C102 reacted with the washing solution (27% loss) converted to NaClO2, according to the formulas: ZNaOH + 2 C102 ___- à NaClOz + NaClO3 + H20 HOCl + NaClOz --- 9 NaClO3 + HCl HCl + NaOH í- Q.)

Claims (6)

H 448 864 Patent claim A process for preparing an aqueous solution suitable for the production of chlorine dioxide and the simultaneous reduction of the chlorine content in a gaseous chlorine dioxide stream obtained by reduction of chlorate, characterized in that the chlorine is removed by reaction with a solution containing sodium hydroxide in an approximate amount of sodium chloride. and sodium chlorate in such a composition that the ratio of chloride to chlorate in the reaction product is about Û, 7: 1 - 5: 1. 2. A process according to claim 1, characterized in that the reaction is carried out at a temperature of about 50-10D ° C and at a pH of about 4-9. 3. A process according to claim 1 or 2, characterized in that the reaction is carried out at a temperature of about 70 ~ 80 ° C and at a pH of about 6-8. 4. A method according to claim 1, characterized in that the amount of sodium chloride in the solution exceeds 75% of the saturation concentration. 5. A method according to claim 1 or 4, characterized in that the amount of sodium chloride in the solution exceeds 90% of the saturation concentration. 6. A process according to any one of claims 1, 4 or 5, characterized in that the solution is so composed that the molar ratio of sodium chloride to sodium chlorate in the reaction product is about 1.0: 1 - 1.3: 1 .